1. Field of the Invention
The invention relates to the field of laser driver circuits, and in particular, to a laser driver for vertical cavity surface emitting laser (VCSEL) arrays.
2. Background Information
A laser device called a VCSEL (Vertical Cavity Surface Emitting Laser) is known. Simply put, a VCSEL is a semiconductor laser made of many layers, e.g., 600, which emits light vertically from a lower surface and in a direction parallel to the direction of its optical cavity, as opposed to an edge-emitting type laser structure. VCSEL""s have advantages over edge-emitting type structures because, for example, the edge-emitting type lasers must be precisely broken or cleaved individually to form each device during manufacturing. However, with VCSEL""s, literally millions of laser devices can be made simultaneously in an etching process.
VCSEL""s are currently some of the smallest lasers being produced. There is a relatively new type of VCSEL in development, the QD-VCSEL. The xe2x80x98QDxe2x80x99 signifies the Quantum Dots which are used in the active layer of this type of VCSEL. The QD-VCSEL promises to achieve even further size reductions.
VCSEL""s have a range of uses. For example, a specially designed VCSEL has been used to create an optical latch or optical state memory, the VCSEL transitioning and latching in the ON state when an optical input is received. Arrays of such VCSEL""s open up possibilities for various massively parallel optical computing applications such as pattern recognition. VCSEL""s have data communications applications as well as would be clear to one skilled in the art, for example, as transmitters in parallel optical links. For more information about VCSEL""s, see, for example, xe2x80x9cLASERS, Harnessing the Atom""s Light,xe2x80x9d Harbison et al., Scientific American Library, 1998, pages 169-177.
VCSEL arrays are commonly manufactured in a common cathode configuration, i.e., with all the laser cathodes connected together. A graph representing the optical power output PO in milliwatts (mW) vs. the current input I in milliamps (mA) for a typical VCSEL is shown in FIG. 1. The VCSEL does not begin lasing until the current through it exceeds a certain laser threshold value, shown as Ith in the figure. The slope of the curve above Ith is commonly referred to in the art as the differential quantum efficiency (DQE) of the VCSEL.
The values of both Ith and DQE are process dependent, that is, they are dependent on manufacturing process variations which are, at present, not completely controllable or predictable. Therefore, a method to adjust the current through the VCSEL to compensate for these variations is required.
A method is known to electrically drive a common cathode array as shown in FIG. 2. See, for example, U.S. Pat. No. 4,709,370, Bednarz et al., Nov. 24, 1987.
In particular, Bednarz et al. (U.S. Pat. No. 4,709,370) discloses (Abstract) a driver for driving the anode of a laser diode which includes a constant-current differential switch comprising NPN transistors. Two sources supply constant current into the node where the differential switch drives the laser diode: one source supplies bias current through the laser diode; the second provides a current substantially equal to the constant current through the differential switch. When the transistor, which drives the laser diode, is turned on by a modulating signal applied to its base electrode, the current from the second source flows through it into the current source at the common emitter junction. When this transistor is turned off, the current from the second source flows through the laser diode, adding to the bias current therethrough. Under these circumstances, the total current through the laser diode exceeds the threshold for lasing action and the laser diode is turned on.
Battjes (U.S. Pat. No. 3,633,120) discloses (Abstract) an amplifier circuit for increasing current gain at high frequencies which includes first and second pairs of transistors, wherein the outputs of the transistor pairs are coupled in parallel while a common input current is provided in series to the four transistors. The circuit substantially doubles the current gain achieved at certain high frequencies. This circuit is referred to herein as an fT doubler.
Returning to the FIG. 2 circuitry, the variable current source I1 supplies a DC current equal to the Ith (threshold) current of the VCSEL. Variable current source I2 supplies a DC current equal to the desired modulation current. Variable current source I3 supplies a DC current equal to the current of I2. When signal A0 is high and signal A1 is low, current from current source I3 is directed through Q0. Since the current from current source I3 is equal to the current from current source I2, only the threshold current from current source I1 will be flowing in the VCSEL at this time.
When signal A0 is low and signal A1 is high, the current from current source I3 is directed through Q1. Therefore, a current equal to the sum of the current from current sources I1 and I2 will be flowing in the VCSEL.
In reality, it is desired that the collector current of Q0 equal the current of I2. Mismatches between I2 and I3, and the base current of Q0, cause the collector current of Q0 to not equal I2. The base current xe2x80x98stealsxe2x80x99 some of the I3 current, since I3 is the emitter current, and the collector current is the emitter current less the base current.
However, a mismatch between current sources I2 and I3 would cause the respective currents to be unequal and lead to problems. Some causes for current source mismatch include process variations/tolerances, Vbe (voltage base to emitter) mismatches in bipolar transistor devices, and VT (threshold voltage) in field effect transistor devices
If the current from current source I3 is greater than the current from current source I2, when Q0 is on and Q1 is off, some of I1 will be diverted away from the VCSEL and through Q0, so that less current will flow through the VCSEL. This causes the low level current flowing in the VCSEL to fall below the threshold current Ith. This will cause a turn on delay, since the VCSEL is not at Ith when the turn on signal is first applied. The turn on delay results in what is referred to as xe2x80x98duty cycle distortionxe2x80x99 in the optical output waveform from the VCSEL, and is a potential source of skew, i.e., a timing misalignment, between channels in a multi-channel system with a VCSEL array.
On the other hand, if the current from current source I3 is less than the current from current source I2, a current flows through the VCSEL greater than Ith when Q0 is on. In this case, the average optical power out of the VCSEL will increase beyond desired levels, possibly causing an over-powering condition. The effect is that a current (I1+(I2xe2x88x92I3) greater than the laser threshold current flows through the laser when it is supposed to be at the threshold.
Therefore, to provide a way to compensate for these possible problems, the current source I3 should be adjustable independent of current source I2.
A common implementation of current source I3 is shown in schematic form in FIG. 3. It includes a reference current source Iref and a current mirror including transistors Q2 and Q3. Current Iref controls current lout from Q3 by the connection with Q2. (Note that in the circuits depicted herein, the current is drawn according to so-called xe2x80x98conventional currentxe2x80x99.)
Typically, VCSEL""s have DQE""s in the range of 0.1 mW/mA to 0.6 mW/mA. This causes a 6 to 1 ratio in modulation current necessary in current sources I2 and I3, since the DQE is directly proportional to the modulation current through the laser provided by the current sources I2 and I3 (recall that the slope of the current curve above Ith is the differential quantum efficiency xe2x80x98DQExe2x80x99).
To handle the large current necessary with a low DQE to obtain the needed output power, transistor Q3 of current source I3 (FIG. 3) needs to be physically large. However, this results in a large equivalent (parasitic) capacitance. FIG. 4 illustrates the current source circuit of FIG. 3 connected as current source I3 of FIG. 2. The large equivalent capacitance of Q3, represented by C0 in FIG. 4, is connected between ground and the common emitter node (the node where the emitters of transistors Q0 and Q1 are connected). Such a capacitance can cause timing problems, as will be explained.
FIG. 5 is a timing chart which illustrates the effect this equivalent capacitance C0 of Q3 has on the driver circuit operation. The signals A0 and A1 are represented as voltages VA0 and VA1, respectively. On switching edges of the signals A0 and A1, the common emitter node voltage Vcom temporarily decreases as shown in the lower trace of FIG. 5.
When the common emitter node voltage Vcom decreases, current must be sinked out of the equivalent capacitance C0. However, the only source for this xe2x80x98deltaxe2x80x99 current is Q3. Therefore, the source current (IA in FIG. 4) temporarily droops causing both IQ0 and IQ1 to droop.
On the other hand, after the switching is completed, current must be sourced back into the equivalent capacitance C0. The only source for this additional current is again the current IA. Therefore, the current through transistor Q0 (IQ0) or through transistor Q1 (IQ1) temporarily increases to provide this current IA. A graphical representation of this phenomena is shown in the timing diagram of FIG. 6.
These temporary decreases and increases in the currents IQ0 and IQ1 due to the equivalent capacitance of current source transistor Q3 introduce an undesirable duty cycle distortion in the output optical waveform of the VCSEL.
Therefore, a need exists for driver circuitry which compensates for this duty cycle distortion.
Copending related application Ser. No. 09/429,282, filed Oct. 28, 1999, entitled xe2x80x9cOPTICAL POWER ADJUSTMENT CIRCUITS FOR PARALLEL OPTICAL TRANSMITTERSxe2x80x9d assigned to the same assignee as the present application, provides an open loop method to adjust the optical power level to compensate for temperature and process variations, particularly, in VCSEL arrays. In an embodiment disclosed therein, optical power adjustment circuitry for parallel optical transmitters is provided which can independently digitally adjust the threshold current and modulation current for each VCSEL in an array, as well as provide a global temperature coefficient for all the VCSEL""s in the array which can be digitally set.
It is, therefore, a principle object of this invention to provide a method and apparatus for a vertical cavity surface emitting laser (VCSEL) driver with low duty cycle distortion.
It is another object of the invention to provide a method and apparatus with digital modulation adjustment.
It is another object of the invention to provide a method and apparatus that solves the above mentioned problems.
These and other objects of the present invention are accomplished by the method and apparatus disclosed herein.
According to an aspect of the invention, a laser driver for use in the modulation of a laser device includes a first means for providing a threshold laser current, a second means for providing a laser current equal to a desired modulation current, and a third means for providing the desired modulation current in response to input modulation signals. The third means is further for reducing duty cycle distortion during the modulation of the laser device.
According to an aspect of the invention, the driver is for modulating the laser device between an on and a threshold/off state.
According to another aspect of the invention, in the threshold/off state, the driver operates so that only the threshold current flows through the laser, and in the on state, a sum of the threshold current and the modulation current flows through the laser.
According to another aspect of the invention, the first means comprises a first variable current source which provides the threshold laser current. The second means comprises a second variable current source which provides the laser current equal to a desired modulation current. The third means comprises an fT doubler circuit which provides the desired modulation current.
According to another aspect of the invention, the fT doubler circuit includes a first and a second differential pair of transistors, and a variable current source. The first and second differential pairs of transistors receive input signals which cause modulation of the laser device. The current source includes two current source transistors, a diode connected transistor, and a reference current source.
According to another aspect of the invention, the laser device is a vertical cavity surface emitting laser (VCSEL), or an edge emitting laser device.
According to another aspect of the invention, the current adjusting means comprises digital to analog converter means for receiving digital adjustment bits and adjusting the respective currents accordingly.
According an aspect of the invention, there is provided a laser driver with digital modulation adjustment which includes a first variable current source which provides a threshold laser current, a second variable current source which provides a laser current equal to a desired modulation current, and an fT doubler circuit which provides the desired modulation current in response to input modulation signals. The fT doubler circuit includes a first and a second differential pair of transistors, and a third variable current source. The first and second differential pairs of transistors receive input signals which cause modulation of the laser device. The third current source includes two first digital to analog converters which provide currents proportional to input bits, to the first and second differential pairs of transistors, and a current mirror circuit including a digital to analog converter (W1/L, W4/L2) and a reference current source.
These and other aspects of the invention will become apparent from the detailed description set forth below.